The present invention relates to a method for joining a longitudinal seam of a flexible packaging and particularly to a method for joining a sealing seam of a tubular bag packaging by means of an ultrasonic applicator. The invention also relates to a longitudinal seal joining device of a tubular bag machine for use with the method.
Methods are already known in which join connections on packagings are produced on horizontal and vertical tubular bag machines by means of conventional ultrasound in the range of 20 to 40 kHz.
After forming the packaging material on the forming shoulder, the film is joined to a packaging material tube by means of the longitudinal seam. After feeding the product, the transverse seams are sealed, the product feed can, however, also take place already during the formation of the tube. The packaging material tube is subsequently divided into individual tubular bags by means of a cutting function.
In the known methods, conventional ultrasonic systems consist of a converter, an optional booster and a sonotrode. The vibration generated in the converter by means of the inverse piezoelectric effect is intensified in the booster and directed into the packaging material via the sonotrode. A standing wave is formed in the resonantly vibrating overall system (FIG. 12). The tools, in particular the sonotrode, must be designed as λ/2 transducers and therefore are subject to geometrical restrictions (FIG. 13). These functional constraints mean that the application on the tubular bag machine has to be oriented to the operating principle of the sonotrode and not to the dimensions of the packaging. The production of the sonotrodes has proven to be complicated and expensive. This is also a reason why this expensive technology is used in the market only for special applications.
The heat generation in the packaging material is based on inner friction and boundary surface friction. In order to fuse the packaging material, a high contact pressure (>200 N) is therefore necessary between sonotrode and packaging material.
The induction of sound into the joining zone generally occurs perpendicularly to the joining surface. A precondition for the heat generation is the use of an energy director as a counterpart to the sonotrode, said energy director defining the working zone of the tool system.
Due to the aforementioned standing wave in the oscillation system, fastening options for the sonotrode are only possible at the nodal points of vibration, where said sonotrode has a zone of the smallest expansion. In this so-called zero line, vibrations should hardly be present. A connection of the sonotrode to corresponding machine elements outside of nodal points of vibration generally leads to a severance and destruction of the coupling point due to the extremely high acceleration forces.
According to the patent publication DT 1704204 A, a method for joining plastic parts based on focused ultrasound is furthermore known. The ultrasonic waves at a frequency of over 100 kHz generated by a vibrator are bundled by means of the vibrator geometry or a focusing assembly and transmitted through a liquid medium. The sound waves directed through the liquid medium excite a membrane into vibration which is pressed onto the plastic parts to be welded.
In addition, the German patent publication DE 10 2008 044 370 A1 describes a device and a method for connecting components by means of focused ultrasound. In this case, the wave guide is formed either by a conical element produced from a metallic solid material or from a liquid- or gel-filled membrane.